1. Field of the Invention
The invention relates to excimer and molecular fluorine laser systems, and particularly including a line-narrowing resonator configuration including fixed or adjustable wavefront curvature compensation.
2. Description of the Related Art
Line-narrowed excimer lasers are applied in the field of photolithography for production of integrated circuits. Achromatic imaging optics for this wavelength region are difficult to produce. For this reason line-narrowed excimer laser radiation is generated for use in photolithographic applications in order to prevent errors caused by chromatic aberrations. Exemplary bandwidths for different imaging systems are tabulated in Table 1 for the excimer laser wavelengths 248 nm (KrF laser), 193 nm (ArF laser), and for the molecular fluorine laser wavelength 157 nm (F2-laser).
Another important beam parameter is the xe2x80x9cspectral purityxe2x80x9d or the wavelength interval that contains, e.g., 95% of the energy of the laser radiation. Excimer lasers may include a planar diffraction grating within its resonator for providing dispersion as a wavelength selector for narrowing the bandwidth of the laser oscillation. To increase the resolution of the grating, a beam expander may be used to reduce the beam divergence. Even when a beam expander is used, the wavefront of the beam in front of or incident upon the grating is generally not planar. The radius of curvature of the wavefront may instead depend on the magnification of the beam expander in the region of, e.g., 400 m . . . 1200 m. The curved wavefront results in a broader spectral linewidth or bandwidth due to the fact that different portions of the curved wavefront strike the planar grating at different angles.
It is recognized in the present invention that it is desired to solve the above-described problem by providing an excimer or molecular fluorine laser resonator having wavefront compensation, and preferably including adjustable wavefront curvature correction, so that the incoming beam wavefront substantially matches the surface of the line-narrowing grating.
In view of the above, an excimer or molecular fluorine laser system is provided including a discharge chamber filled with a gas mixture at least including molecular fluorine and a buffer gas, multiple electrodes within the discharge chamber connected to a power supply circuit for energizing the gas mixture, a resonator for generating a laser beam, a line-narrowing unit within the resonator preferably including multiple optical components such as a beam expander and a grating, and a wavefront compensating optic for adjusting the curvature of the wavefront of the beam for compensating wavefront distortions and thereby enhancing the spectral purity of the beam.
The wavefront compensating optic is preferably and most advantageously disposed between the beam expander and the grating of the line-narrowing unit. In alternative embodiments, the wavefront compensating optic is otherwise positioned in the laser resonator such as between the discharge chamber and the line-narrowing unit, or the discharge chamber may be disposed between the line-narrowing unit and the wavefront compensating optic. In accordance with the preferred embodiment, the wavefront compensating optic may be sealably disposed between adjacent compartments of a multi-compartment enclosure, wherein each compartment includes at least one optic of the line-narrowing unit.
The wavefront compensating optic may be a plate, such as a null lens. One or both surfaces of the null lens may be adjustable and/or have an adjustable curvature for controlling the wavefront distortion compensation. The wavefront compensating optic may sealably enclose the gas mixture as a window of the discharge chamber.
A multi-compartment enclosure may be included having at least one optical component of the line-narrowing unit within each compartment. An atmosphere within at least one compartment may be preferably controlled for controlling the spectral purity of the beam by controlling an amount of wavefront distortion compensation. A wavefront compensating optic may be sealably disposed between adjacent compartments.